The present invention relates to an AC power control apparatus and, more particularly, to an AC power control apparatus for heating a heater for a glass plate so as to prevent the glass plate from fogging and icing caused by a difference between room and atmospheric temperatures.
A heater is embedded in a windshield in front of a pilot seat in an airplane. A current is supplied to the heater and the heater is heated, thereby preventing the windshield from fogging and icing.
This current is supplied from an AC power source arranged inside the airplane. The value of the current must be adjusted depending on the atmospheric temperature, and thus phase control of the current is performed by a thyristor circuit in a conventional AC power control apparatus.
In a conventional AC power control apparatus, however, an AC waveform of the power source current is discontinuous and noise is undesirably generated. In order to eliminate noise, the current on/off operation may be performed at a zero level timing of the current.
In order to perform zero level control of the AC waveform, however, zero level detection must be performed. However, conventional zero level detection requires a complicated circuit, thus failing to satisfy economical requirements and circuit reliability.
Furthermore, when an over-current is supplied to the heater, the over-current is detected and controlled to be cut off from the heater.
In order to detect an AC over-current, however, an AC waveform must be rectified, the rectified signal must be amplified, and finally the level of the amplified signal must be detected. Therefore, the circuit arrangement is indispensably complicated, and DC amplification must be performed, thus causing circuit instability. In addition, an expensive circuit must be undesirably used.
When phase control of the AC current is performed by a thyristor circuit, an AC power source voltage is not matched with a rated voltage of the heater. For this reason, the power source voltage must be transformed to the rated voltage by a voltage transformer. In this case, if a heater current on/off interval is short, an apparatus using a voltage transformer must supply to a voltage transformer a voltage having a polarity opposite to that in the current cutoff state so as to prevent magnetic saturation of an iron core. In addition, in order to eliminate noise, the current is subjected to zero level control. However, since high stability of the power source frequency inside the airplane is not assured, it is difficult to generate a trigger signal to perform accurate current turn on or energization control.